Nailing down the risks and benefits of eating fish is easier said than done. Eating the omega-3 fatty acids found in fish has its benefits for the developing brain, but pregnant women and new moms are advised to eat no more than 12 ounces of seafood per week due to concerns about mercury.

Studies suggest that the benefits of omega-3s outweigh the risks of mercury, especially if women avoid species such as shark, swordfish, king mackerel and tilefish, all of which are high in mercury.

The Martek Biosciences Corporation manufactures DHA from algae, which is where fish get their DHA, explains J. Casey Lippmeier, Martek's senior scientist.

The algae are eaten by smaller marine life such as copepods, "and those are eaten by slightly larger fish," says Lippmeier. The DHA gets passed along the food chain, all the way up to the biggest fish.

In the wild, big fish that have lots of DHA may also have contaminants such as mercury or PCBs in their tissue. So, as Lippmeier sees it, Martek is producing a pure supply of DHA by fermenting algae in big, steel tanks.

"We know all the ingredients going into it, so we can assure all the ingredients are coming out of it," Lippmeier says.

Infant Formula and Fish Oil

When Martek's biologists first came up with the idea of growing algae to produce DHA supplements, no one seemed to notice.

But about five years ago, manufacturers of infant milk formula began adding Martek's DHA to the formula. The decision was based on clinical trials that found infants, particularly preemies, who were fed a DHA supplement did better on cognitive tests than babies who were fed a regular formula. The babies, for instance, were better able to recognize a new face compared with one they had seen before.

The infant formula studies clearly supported babies getting more DHA. But the research also found that the benefits of the supplement did not persist as the infants became toddlers.

This finding led to more questions. It's possible that the benefits of DHA supplementation are short-lived.

"It's also possible," says researcher Emily Oken of Harvard Medical School, "that prenatal intake is more important than post-natal intake."

In other words, what a woman eats during pregnancy may be more important than what the infant eats after birth.

DHA and the Brain

For pregnant women and developing babies, DHA is crucial. It's found in every organ of the body as well as in muscle tissue and in the brain. DHA makes brain membranes flexible and optimizes electrical signaling.

Animal studies show that when DHA is drastically reduced in the brain, processing slows.

"From neuron to neuron, or from the retina to the brain, those signals will still go," says Norman Salem, a neurobiologist with the National Institutes of Health. "But they may be slower and not as intense."

Other Sources of Omega-3 Fatty Acids

Humans have always consumed omega-3s. Eggs contain some DHA, and organ meats such as liver and brains, are abundant sources, too.

Nuts and seeds such as walnuts, flax-seed and Canola oil contain omega-3s, though the body has to convert these plant sources from alpha-linolenic Acid (ALA) to DHA.

Since fish are loaded with a lot of DHA and protein, Harvard's Emily Oken has made fish the focus of her work.

Oken and her colleagues are among the first to closely track the risks and the benefits of fish-eating in a group of Americans. Eight years ago, they recruited hundreds of pregnant women who shared details of their diets and gave blood samples so scientists could see exactly how much mercury was in their blood.

When the children of these women turned 3, researchers evaluated them using the Peabody Picture Vocabulary Test, which correlates with verbal IQ.

Fish intake was associated with a two-point higher score. But children whose mothers had the highest blood-mercury levels (in the top 10 percent) scored about four points lower on the test. Based on these results, "the best benefit comes with eating fish that are low in mercury," Oken says.

While the cognitive testing differences are measurable, there are not huge swings.

"We think these kinds of exposure are not likely to take kids who would have been a genius and make them below average," Oken says.

These are very subtle effects for any individual child. Across the whole population, however, this level of change could influence the number of children on the threshold of being mentally retarded — or the number of children within reach of being considered academically gifted.

Oken's research is ongoing. She and her colleagues plan to continue evaluating the children through the school years to better understand how these associations between fish consumption and mental development hold up.

Fish FAQ: What You Need to Know About Mercury

As a rule, the bigger the fish, the higher the mercury content.
Environmental Protection Agency
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Environmental Protection Agency

What is mercury?

Mercury is an element that occurs naturally in the environment. It can also be released into the atmosphere through industrial pollution. Because mercury dissolves easily in water, it accumulates at the bottom of bodies of water.

What is methylmercury? How does it enter the food chain?

In environments without oxygen, mercury combines with carbon to become methylmercury, a highly toxic compound. Bottom-dwelling fish consume methylmercury particles on the ocean floor and are then eaten by larger fish. Large fish such as sharks, swordfish and king mackerel contain the largest amounts of methylmercury because they're higher up in the food chain.

Why is methylmercury dangerous?

Methylmercury is toxic. It can harm the immune system, the gastrointestinal system, and even the genetic code. It attacks the central nervous system. High levels of exposure will result in brain damage and eventual death.

Most people already have some levels of methylmercury in their system from eating fish, but effects of mercury exposure depend on factors like age and amount of exposure. Trace amounts of methylmercury are excreted over time.

Should I stop eating fish?

No. The health benefits of fish outweigh the side effects of methylmercury exposure. The FDA recommends the following:

1. Do not eat shark, swordfish, king mackerel or tilefish. These fish typically contain high levels of mercury.

2. It's not risky to eat up to 12 ounces a week of fish and shellfish that are lower in mercury, such as shrimp, canned light tuna, salmon, pollock, and catfish.

3. You may also eat up to 6 ounces of albacore tuna, but the level of mercury is higher in albacore tuna than in light tuna, so substitute other types of fish for your other 6 ounces.

Why should pregnant women avoid methylmercury?

Developing embryos are particularly sensitive to methylmercury, so pregnant women and women trying to become pregnant are advised to avoid all large fish. They should also eat no more than one six-ounce serving of tuna a week, and should limit overall consumption of fish to no more than 12 ounces a week, according to the FDA.

What if I want to catch and eat my own fish?

Contact your local health department about mercury advisories in the waters where you plan on fishing. Fish that is caught recreationally may be OK, depending on whether your local waters have higher or lower mercury levels. If no advice is available, eat up to one meal a week of fish from local waters, but don't consume other fish that week.